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使用&amp;&amp;表示
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使用&amp;&amp;表示
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将左值转换为对应的右值使用std::move
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使用&amp;&amp;表示
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将左值转换为对应的右值使用std::move
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      <h1 class="post-title">右值引用</h1>

      <div class="post-meta">
        <span class="post-time"> 2019-12-09 </span>
        
        
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    <div class="post-toc" id="post-toc">
  <h2 class="post-toc-title">Contents</h2>
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  <ul>
    <li><a href="#右值引用">右值引用</a>
      <ul>
        <li><a href="#移动构造和移动赋值">移动构造和移动赋值</a></li>
      </ul>
    </li>
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    <div class="post-content">
      <h1 id="右值引用">右值引用</h1>
<ul>
<li>使用&amp;&amp;表示</li>
<li>只能绑定到一个将要销毁的对象</li>
<li>左值持久，右值短暂</li>
<li>将左值转换为对应的右值使用std::move</li>
</ul>
<div class="highlight"><div class="chroma">
<table class="lntable"><tr><td class="lntd">
<pre class="chroma"><code><span class="lnt">1
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</span><span class="lnt">5
</span><span class="lnt">6
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<td class="lntd">
<pre class="chroma"><code class="language-cpp" data-lang="cpp"><span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">42</span><span class="p">;</span>
<span class="kt">int</span> <span class="o">&amp;</span><span class="n">r</span> <span class="o">=</span> <span class="n">i</span><span class="p">;</span>     <span class="c1">//左值引用，ok
</span><span class="c1"></span><span class="kt">int</span> <span class="o">&amp;&amp;</span><span class="n">rr</span> <span class="o">=</span> <span class="n">i</span><span class="p">;</span>   <span class="c1">//错误，不能把右值引用绑定到左值上
</span><span class="c1"></span><span class="kt">int</span> <span class="o">&amp;</span><span class="n">r2</span> <span class="o">=</span> <span class="n">i</span><span class="o">*</span><span class="mi">42</span><span class="p">;</span> <span class="c1">//错误，i*42是一个右值
</span><span class="c1"></span><span class="k">const</span> <span class="kt">int</span> <span class="o">&amp;</span><span class="n">r3</span> <span class="o">=</span> <span class="n">i</span><span class="o">*</span><span class="mi">42</span><span class="p">;</span>   <span class="c1">//ok，const引用可以绑定到右值
</span><span class="c1"></span><span class="kt">int</span> <span class="o">&amp;&amp;</span><span class="n">rr2</span> <span class="o">=</span> <span class="n">i</span><span class="o">*</span><span class="mi">42</span><span class="p">;</span>   <span class="c1">//ok，右值引用绑定右值
</span></code></pre></td></tr></table>
</div>
</div><h2 id="移动构造和移动赋值">移动构造和移动赋值</h2>
<p>例如这样一个类StrVec中有移动构造和移动赋值，使用&amp;&amp;作为参数</p>
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<td class="lntd">
<pre class="chroma"><code class="language-cpp" data-lang="cpp"><span class="c1">// StrVec class, a class like vector but only store string as element
</span><span class="c1">// CppPrimer 13.5
</span><span class="c1">// lcl 20190712
</span><span class="c1"></span>
<span class="cp">#include</span> <span class="cpf">&lt;iostream&gt;</span><span class="cp">
</span><span class="cp">#include</span> <span class="cpf">&lt;string&gt;</span><span class="cp">
</span><span class="cp">#include</span> <span class="cpf">&lt;memory&gt;</span><span class="cp">
</span><span class="cp"></span><span class="k">using</span> <span class="k">namespace</span> <span class="n">std</span><span class="p">;</span>

<span class="k">class</span> <span class="nc">StrVec</span> <span class="p">{</span>
    <span class="k">public</span><span class="o">:</span>
    <span class="n">StrVec</span><span class="p">()</span><span class="o">:</span> <span class="n">elements</span><span class="p">(</span><span class="k">nullptr</span><span class="p">),</span> <span class="n">first_free</span><span class="p">(</span><span class="k">nullptr</span><span class="p">),</span> <span class="n">cap</span><span class="p">(</span><span class="k">nullptr</span><span class="p">){}</span>
    <span class="n">StrVec</span><span class="p">(</span><span class="k">const</span> <span class="n">StrVec</span> <span class="o">&amp;</span><span class="p">);</span> <span class="c1">// copy constructor
</span><span class="c1"></span>    <span class="n">StrVec</span><span class="p">(</span><span class="n">StrVec</span> <span class="o">&amp;&amp;</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span> <span class="c1">// move constructor
</span><span class="c1"></span>    <span class="n">StrVec</span> <span class="o">&amp;</span><span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">StrVec</span><span class="o">&amp;</span><span class="p">);</span> <span class="c1">// copy assignment
</span><span class="c1"></span>    <span class="n">StrVec</span> <span class="o">&amp;</span><span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="n">StrVec</span><span class="o">&amp;&amp;</span><span class="p">)</span> <span class="k">noexcept</span><span class="p">;</span> <span class="c1">//move assignment
</span><span class="c1"></span>    <span class="o">~</span><span class="n">StrVec</span><span class="p">();</span>
    <span class="kt">void</span> <span class="nf">push_back</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&amp;</span><span class="p">);</span>
    <span class="n">size_t</span> <span class="nf">size</span><span class="p">()</span> <span class="k">const</span> <span class="p">{</span><span class="k">return</span> <span class="n">first_free</span> <span class="o">-</span> <span class="n">elements</span><span class="p">;}</span>
    <span class="n">size_t</span> <span class="nf">capacity</span><span class="p">()</span> <span class="k">const</span> <span class="p">{</span><span class="k">return</span> <span class="n">cap</span> <span class="o">-</span> <span class="n">elements</span><span class="p">;}</span>
    <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">*</span><span class="n">begin</span><span class="p">()</span> <span class="k">const</span> <span class="p">{</span> <span class="k">return</span> <span class="n">elements</span><span class="p">;}</span>
    <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">*</span><span class="n">end</span><span class="p">()</span> <span class="k">const</span> <span class="p">{</span> <span class="k">return</span> <span class="n">first_free</span><span class="p">;}</span>
    <span class="k">private</span><span class="o">:</span>
    <span class="n">std</span><span class="o">::</span><span class="n">allocator</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span> <span class="n">alloc</span><span class="p">;</span>
    <span class="kt">void</span> <span class="nf">chk_n_alloc</span><span class="p">()</span>
    <span class="p">{</span>
        <span class="k">if</span><span class="p">(</span><span class="n">size</span><span class="p">()</span><span class="o">==</span><span class="n">capacity</span><span class="p">())</span> 
        <span class="n">reallocate</span><span class="p">();</span>
    <span class="p">}</span>
    <span class="n">std</span><span class="o">::</span><span class="n">pair</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">*</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">*&gt;</span> <span class="n">alloc_n_copy</span>
        <span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">*</span><span class="p">,</span> <span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">*</span><span class="p">);</span>
    <span class="kt">void</span> <span class="nf">free</span><span class="p">();</span>
    <span class="kt">void</span> <span class="nf">reallocate</span><span class="p">();</span>
    <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">*</span><span class="n">elements</span><span class="p">;</span>   <span class="c1">//point to first element
</span><span class="c1"></span>    <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">*</span><span class="n">first_free</span><span class="p">;</span>    <span class="c1">//point to first free element
</span><span class="c1"></span>    <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">*</span><span class="n">cap</span><span class="p">;</span>   <span class="c1">//point to rear;
</span><span class="c1"></span><span class="p">};</span>

<span class="kt">void</span> <span class="n">StrVec</span><span class="o">::</span><span class="n">push_back</span><span class="p">(</span><span class="k">const</span> <span class="n">string</span><span class="o">&amp;</span> <span class="n">s</span><span class="p">)</span>
<span class="p">{</span>
    <span class="n">chk_n_alloc</span><span class="p">();</span>
    <span class="n">alloc</span><span class="p">.</span><span class="n">construct</span><span class="p">(</span><span class="n">first_free</span><span class="o">++</span><span class="p">,</span> <span class="n">s</span><span class="p">);</span>
<span class="p">}</span>

<span class="n">pair</span><span class="o">&lt;</span><span class="n">string</span><span class="o">*</span><span class="p">,</span> <span class="n">string</span><span class="o">*&gt;</span>
<span class="n">StrVec</span><span class="o">::</span><span class="n">alloc_n_copy</span><span class="p">(</span><span class="k">const</span> <span class="n">string</span> <span class="o">*</span><span class="n">b</span><span class="p">,</span> <span class="k">const</span> <span class="n">string</span><span class="o">*</span> <span class="n">e</span><span class="p">)</span>
<span class="p">{</span>
    <span class="k">auto</span> <span class="n">data</span> <span class="o">=</span> <span class="n">alloc</span><span class="p">.</span><span class="n">allocate</span><span class="p">(</span><span class="n">e</span><span class="o">-</span><span class="n">b</span><span class="p">);</span>
    <span class="k">return</span> <span class="p">{</span><span class="n">data</span><span class="p">,</span> <span class="n">uninitialized_copy</span><span class="p">(</span><span class="n">b</span><span class="p">,</span><span class="n">e</span><span class="p">,</span><span class="n">data</span><span class="p">)};</span>
<span class="p">}</span>

<span class="kt">void</span> <span class="n">StrVec</span><span class="o">::</span><span class="n">free</span><span class="p">()</span>
<span class="p">{</span>
    <span class="k">if</span><span class="p">(</span><span class="n">elements</span><span class="p">){</span>
        <span class="k">for</span><span class="p">(</span><span class="k">auto</span> <span class="n">p</span> <span class="o">=</span> <span class="n">first_free</span><span class="p">;</span> <span class="n">p</span><span class="o">!=</span><span class="n">elements</span><span class="p">;</span><span class="n">alloc</span><span class="p">.</span><span class="n">destroy</span><span class="p">(</span><span class="o">--</span><span class="n">p</span><span class="p">));</span>
        <span class="n">alloc</span><span class="p">.</span><span class="n">deallocate</span><span class="p">(</span><span class="n">elements</span><span class="p">,</span> <span class="n">cap</span><span class="o">-</span><span class="n">elements</span><span class="p">);</span>
    <span class="p">}</span>
<span class="p">}</span>

<span class="n">StrVec</span><span class="o">::</span><span class="n">StrVec</span><span class="p">(</span><span class="k">const</span> <span class="n">StrVec</span><span class="o">&amp;</span><span class="n">s</span><span class="p">)</span>
<span class="p">{</span>
    <span class="k">auto</span> <span class="n">newdata</span> <span class="o">=</span> <span class="n">alloc_n_copy</span><span class="p">(</span><span class="n">s</span><span class="p">.</span><span class="n">begin</span><span class="p">(),</span> <span class="n">s</span><span class="p">.</span><span class="n">end</span><span class="p">());</span>
    <span class="n">elements</span> <span class="o">=</span> <span class="n">newdata</span><span class="p">.</span><span class="n">first</span><span class="p">;</span>
    <span class="n">first_free</span> <span class="o">=</span> <span class="n">cap</span> <span class="o">=</span> <span class="n">newdata</span><span class="p">.</span><span class="n">second</span><span class="p">;</span>
<span class="p">}</span>

<span class="n">StrVec</span><span class="o">::~</span><span class="n">StrVec</span><span class="p">()</span>
<span class="p">{</span>
    <span class="n">free</span><span class="p">();</span>
<span class="p">}</span>

<span class="n">StrVec</span><span class="o">&amp;</span> <span class="n">StrVec</span><span class="o">::</span><span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">StrVec</span> <span class="o">&amp;</span><span class="n">rhs</span><span class="p">)</span>
<span class="p">{</span>
    <span class="c1">// 先分配大小相同的内存拷贝，再释放掉原data
</span><span class="c1"></span>    <span class="k">auto</span> <span class="n">data</span> <span class="o">=</span> <span class="n">alloc_n_copy</span><span class="p">(</span><span class="n">rhs</span><span class="p">.</span><span class="n">begin</span><span class="p">(),</span> <span class="n">rhs</span><span class="p">.</span><span class="n">end</span><span class="p">());</span>
    <span class="n">free</span><span class="p">();</span>
    <span class="n">elements</span> <span class="o">=</span> <span class="n">data</span><span class="p">.</span><span class="n">first</span><span class="p">;</span>
    <span class="n">first_free</span> <span class="o">=</span> <span class="n">cap</span> <span class="o">=</span> <span class="n">data</span><span class="p">.</span><span class="n">second</span><span class="p">;</span>
    <span class="k">return</span> <span class="o">*</span><span class="k">this</span><span class="p">;</span>
<span class="p">}</span>

<span class="kt">void</span> <span class="n">StrVec</span><span class="o">::</span><span class="n">reallocate</span><span class="p">()</span>
<span class="p">{</span>
    <span class="k">auto</span> <span class="n">newcapacity</span> <span class="o">=</span> <span class="n">size</span><span class="p">()</span> <span class="o">?</span> <span class="mi">2</span> <span class="o">*</span> <span class="n">size</span><span class="p">()</span> <span class="o">:</span> <span class="mi">1</span><span class="p">;</span>
    <span class="k">auto</span> <span class="n">newdata</span> <span class="o">=</span> <span class="n">alloc</span><span class="p">.</span><span class="n">allocate</span><span class="p">(</span><span class="n">newcapacity</span><span class="p">);</span>
    <span class="k">auto</span> <span class="n">dest</span> <span class="o">=</span> <span class="n">newdata</span><span class="p">;</span>
    <span class="k">auto</span> <span class="n">elem</span> <span class="o">=</span> <span class="n">elements</span><span class="p">;</span>
    <span class="k">for</span><span class="p">(</span> <span class="n">size_t</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">i</span><span class="o">!=</span><span class="n">size</span><span class="p">();</span><span class="o">++</span><span class="n">i</span><span class="p">)</span>
        <span class="c1">// 使用了std::move，从而会调用移动构造函数
</span><span class="c1"></span>        <span class="n">alloc</span><span class="p">.</span><span class="n">construct</span><span class="p">(</span><span class="n">dest</span><span class="o">++</span><span class="p">,</span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="o">*</span><span class="n">elem</span><span class="o">++</span><span class="p">));</span>
    <span class="n">free</span><span class="p">();</span>

    <span class="n">elements</span> <span class="o">=</span> <span class="n">newdata</span><span class="p">;</span>
    <span class="n">first_free</span> <span class="o">=</span> <span class="n">dest</span><span class="p">;</span>
    <span class="n">cap</span> <span class="o">=</span> <span class="n">elements</span> <span class="o">+</span> <span class="n">newcapacity</span><span class="p">;</span>
<span class="p">}</span>

<span class="n">StrVec</span><span class="o">::</span><span class="n">StrVec</span><span class="p">(</span><span class="n">StrVec</span> <span class="o">&amp;&amp;</span><span class="n">s</span><span class="p">)</span> <span class="k">noexcept</span> <span class="c1">//移动操作不抛出任何异常
</span><span class="c1"></span>    <span class="o">:</span><span class="n">elements</span><span class="p">(</span><span class="n">s</span><span class="p">.</span><span class="n">elements</span><span class="p">),</span> <span class="n">first_free</span><span class="p">(</span><span class="n">s</span><span class="p">.</span><span class="n">first_free</span><span class="p">),</span> <span class="n">cap</span><span class="p">(</span><span class="n">s</span><span class="p">.</span><span class="n">cap</span><span class="p">)</span>
<span class="p">{</span>
    <span class="c1">// 令s进入这样的状态，对其进行析构是安全的？
</span><span class="c1"></span>    <span class="c1">// 这里令元素为空，析构函数将不能释放空间
</span><span class="c1"></span>    <span class="c1">// 因为元素已经转给移动后的对象，空间不应该释放，所以令他们为空
</span><span class="c1"></span>    <span class="n">s</span><span class="p">.</span><span class="n">elements</span> <span class="o">=</span> <span class="n">s</span><span class="p">.</span><span class="n">first_free</span> <span class="o">=</span> <span class="n">s</span><span class="p">.</span><span class="n">cap</span> <span class="o">=</span> <span class="k">nullptr</span><span class="p">;</span>
<span class="p">}</span>

<span class="c1">// 使用noexcept告知编译器不会抛出异常
</span><span class="c1"></span>
<span class="n">StrVec</span><span class="o">&amp;</span> <span class="n">StrVec</span><span class="o">::</span><span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="n">StrVec</span> <span class="o">&amp;&amp;</span><span class="n">rhs</span><span class="p">)</span> <span class="k">noexcept</span>
<span class="p">{</span>
    <span class="c1">// 处理自赋值的情况
</span><span class="c1"></span>    <span class="k">if</span><span class="p">(</span><span class="k">this</span> <span class="o">!=</span> <span class="o">&amp;</span><span class="n">rhs</span><span class="p">){</span>
        <span class="n">free</span><span class="p">();</span>
        <span class="n">elements</span> <span class="o">=</span> <span class="n">rhs</span><span class="p">.</span><span class="n">elements</span><span class="p">;</span>
        <span class="n">first_free</span> <span class="o">=</span> <span class="n">rhs</span><span class="p">.</span><span class="n">first_free</span><span class="p">;</span>
        <span class="n">cap</span> <span class="o">=</span> <span class="n">rhs</span><span class="p">.</span><span class="n">cap</span><span class="p">;</span>
        <span class="n">rhs</span><span class="p">.</span><span class="n">elements</span> <span class="o">=</span> <span class="n">rhs</span><span class="p">.</span><span class="n">first_free</span> <span class="o">=</span> <span class="n">rhs</span><span class="p">.</span><span class="n">cap</span> <span class="o">=</span> <span class="k">nullptr</span><span class="p">;</span>
    <span class="p">}</span>
<span class="p">}</span>

<span class="c1">// test
</span><span class="c1"></span><span class="kt">int</span> <span class="nf">main</span><span class="p">(</span><span class="kt">int</span> <span class="n">argc</span><span class="p">,</span> <span class="kt">char</span><span class="o">*</span> <span class="n">argv</span><span class="p">[])</span>
<span class="p">{</span>
    <span class="n">StrVec</span> <span class="n">sv</span><span class="p">,</span><span class="n">sv1</span><span class="p">;</span>
    <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="n">sv</span><span class="p">.</span><span class="n">size</span><span class="p">()</span> <span class="o">&lt;&lt;</span> <span class="s">&#34; &#34;</span>
              <span class="o">&lt;&lt;</span> <span class="n">sv</span><span class="p">.</span><span class="n">capacity</span><span class="p">()</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
    <span class="n">sv</span><span class="p">.</span><span class="n">push_back</span><span class="p">(</span><span class="s">&#34;first line&#34;</span><span class="p">);</span>
    <span class="n">sv</span><span class="p">.</span><span class="n">push_back</span><span class="p">(</span><span class="s">&#34;second_line&#34;</span><span class="p">);</span>
    <span class="n">sv1</span> <span class="o">=</span> <span class="n">sv</span><span class="p">;</span>
    <span class="n">std</span><span class="o">::</span><span class="n">cout</span> <span class="o">&lt;&lt;</span> <span class="n">sv1</span><span class="p">.</span><span class="n">size</span><span class="p">()</span> <span class="o">&lt;&lt;</span> <span class="s">&#34; &#34;</span>
              <span class="o">&lt;&lt;</span> <span class="n">sv1</span><span class="p">.</span><span class="n">capacity</span><span class="p">()</span> <span class="o">&lt;&lt;</span> <span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
<span class="p">}</span>
</code></pre></td></tr></table>
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        2019-12-09
        
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